It is often desirable to protect sensitive loads from excessive AC voltage, current, frequency, temperature, mechanical limit, or other conditions which may lead to permanent damage to the load, especially when regularly operating at or near the rated load voltage or current. It is also desirable to allow permissible AC voltages and currents to pass to the load without significant attenuation, distortion, or filtration. Due to the variety of loads driven from AC sources, voltage and current protection limits vary significantly, thereby requiring protective circuitry that can support many different trip thresholds. A loudspeaker represents a sensitive load that regularly operates at or near its rated AC power and presents challenges to the art of AC power protection, clue to the fact that loudspeaker transducers typically can handle large power levels for a short duration and reduced power levels for longer durations. In other words, the time duration of the overage is critical to monitor for protective purposes; power limiting too soon will not allow full utilization of the transducer, while power limiting too late will result in permanent damage to the transducer. Additionally, these time duration coefficients change with the transducer temperature and environmental conditions. Protecting sensitive loads from excessive AC power can be a difficult task and many existing techniques have failed to adequately protect the load from all overage conditions while allowing permissible voltages and currents to pass unaltered.
In addition to AC protection, there are other situations where it is desirable to continuously attenuate an AC voltage that is powering a known load, such as reducing the level of an AC signal powering a loudspeaker. Some loudspeaker applications require AC power attenuation to control the output level of the loudspeaker. For example, consider a simple audio system consisting of four loudspeakers connected in a parallel wiring configuration. Situations may arise where reducing the level of one of the four loudspeakers is desirable. This may be a distributed multi-zoned application or a line array application where several elements within an array need to be turned down. Unfortunately, attenuating these high power signals has been costly, inefficient, and resulted in signal degradation. Another application for continuous AC attenuation is within lighting dimmers. Existing AC attenuators used for lighting applications have focused on attenuation of standard line voltage sinusoids, i.e. 120-240VAC at 50-60 Hz, with no effort to address complex signals spanning the entire audio bandwidth, 20 Hz to 20 kHz. Furthermore, AC dimmers have not made efforts to incorporate protective monitoring circuitry, leaving the lamp or load unprotected against AC overage conditions.
Existing circuits designed to protect sensitive loads from AC power overages have generally used a combination of actuating devices and attenuating devices. Actuating devices have been incorporated to actuate, or change electrical conductive state, during over-voltage or over-current conditions, while attenuating devices have been used to attenuate, or reduce the unwanted voltage or current. Some devices are self-actuating and attenuating devices due to their inherent ability to actuate, or change electrical states when certain conditions have been met, as well as introduce an attenuation once actuated. An example of a self-actuating and attenuating device is a Transient Voltage Suppression diode (TVS, comparable to a large zener diode), which can be used to self-commutate, or actuate, and effectively clamp the voltage at a pre-determined threshold by shunting excess current to ground or neutral. Bi-directional TVS diodes have been specifically developed for use within AC or bi-polar DC systems. A Metal Oxide Varistor (commonly referred to as a MOV) is another commonly used self-actuating and attenuating device that operates similarly to a bi-directional TVS diode. Unfortunately, self-actuating devices such as TVS diodes, MOVs, thermistors, etc. are not adjustable and actuation thresholds can vary significantly depending on ambient temperature and/or production tolerances. Another actuating and attenuating device commonly found in loudspeaker transducer protection circuitry is a lamp. Typically employed in a series configuration, as shown in FIG. 2B, a lamp is considered self-actuating in that the filament will not heat (burn) until enough current is flowing. Yet, once an adequate current is established, the filament lights and its impedance increases, thus allowing power attenuation/dissipation in the form of light and heat. Unfortunately, lamps also have several deficiencies such as filament damage, insertion loss, nominal maximum impedance, light output, and excessive heat generation.
Other protection designs have opted for devices that operate solely as actuators or circuit interrupters due to their ability to introduce an open circuit condition when tripped. The most common self-actuating, circuit interruptive device is a fuse, wherein a specially sized and formed conductor is designed to burn away in the presence of an over-current scenario. Fuses are relatively inexpensive and somewhat effective; however, they are designed with fixed trip thresholds, they have predetermined response times, and once blown, are permanently destroyed, i.e. they must be physically replaced. For most sensitive load applications, fuses are not acceptable due to excessive response time wherein the load may sustain permanent damage. Specialized Positive Temperature Coefficient thermistors (PTCs) have been developed to address the permanent destruction issue common to fuses or fusible links, but they fail to solve the fixed threshold and response time problems. Other actuating devices, such as relays, are considered controlled actuating devices because they require triggering circuitry to control the actuation. Controlled actuation is desirable due to the ability to easily change or program the voltage or current thresholds that result in actuation, but many of the electromechanical actuators, such as relays, suffer from limited response time. For example, a standard power relay has a typical turn-on time of approximately five milliseconds, and in some scenarios, this lag in actuation can result in damage to a sensitive load. Another important aspect of controlled actuation is the circuitry used to control or trigger the actuator. In the art of loudspeaker protection, the circuitry controlling the actuator/s has been implemented with non-programmable components, such as resistors, capacitors, and zener diodes; no effort has been made to utilize a microprocessor based device for actuation control. While non-programmable actuation control circuits are effective, they do not allow threshold and time coefficient adjustment without changing circuit component values (resistors and capacitors). This implementation fundamentally requires differing control circuits for each desired combination of trip thresholds and time coefficients.
AC power attenuation has been another key aspect of implementing high-performance protection circuits for sensitive loads. Once the actuation device has tripped, an attenuating device can be used to reduce the AC power to a safe level. Previous designs have incorporated power attenuators that dissipate unwanted power in the form of heat and in some cases light energy as well. Attenuating devices can be grouped as variable attenuators or fixed attenuators. A resistor with constant impedance would be considered a fixed attenuator, while a lamp with filament heat-dependant impedance would be considered a limited-range variable attenuator. A common device employed for AC power attenuation has been the resistor. Power resistors come in many different shapes, sizes, and constructions (thick film, wire-wound, etc.) and typically have been packaged within cases that allow significant thermal dissipation. While resistive attenuation has achieved the desired result, the drawbacks are significant. Excessive power dissipation requires large, costly resistors, and in some cases specialized heat sinks and/or liquid cooled apparatus are required to dissipate the thermal energy. In the art of loudspeaker protection and attenuation, resistors and lamps are the two most common devices for high power attenuation; no attempts have been made to use transistorized, programmable attenuators.
Existing circuits designed to continuously attenuate an AC power signal driving a loudspeaker, such as circuits designed to lower the amplitude of a signal in a distributed or line array system, have taken one of two approaches: resistive attenuation or transformer voltage step-down. Unfortunately, these existing techniques of continuously attenuating or stepping-down the AC power signal suffer from several significant problems. Circuitry designed to attenuate an AC power signal using resistive attenuation suffers from excessive power loss, thereby requiring large, high-power resistive elements that produce significant heat and can be costly. Alternatively, approaches using the transformer step-down approach benefit from much lower loss (typically 1-2 dB insertion loss), but introduce the following drawbacks to the system: (1) significant physical size and weight due to low frequency magnetic core, (2) frequency response degradation (low frequencies are rolled off), (3) costly as power increases, (4) transformer core saturation problems limit the effective usefulness to low power applications (typically 100 W or less), (5) fixed number of secondary windings “taps” (typically 4) does not allow fine amplitude control. These problems have been prevalent for decades, and no improved solution has been established in situations where continuous attenuation is required.
To illustrate the deficiencies found in many of the existing AC power protection circuits, several examples are presented, wherein the load is a sensitive inductive load, a loudspeaker, having dynamic power handling characteristics that change with duration and overage. Referring to FIG. 1, two aspects of a typical loudspeaker power handling performance are presented; required attenuation and time duration. Typical loudspeakers have a rated power handling specification, below which the transducer will operate without damage illustrated by the dotted line, 44. The left axis, 40, corresponds to an attenuation value in decibels (dB). The solid line, 42, represents the loudspeakers required attenuation to sustain proper operation without damage. The right vertical axis, 46, corresponds to time in seconds (sec). The dashed line, 48, represents the loudspeakers power handling as a function of time duration in seconds. The common horizontal axis, 50, represents increasing power. As evident in FIG. 1A, the transducer requires increasing attenuation as the input power level exceeds the rated power of the loudspeaker. Also, as the input power increases, the duration of time within which the transducer will operate without damage steadily decreases. Effective protection should seek to provide adequate attenuation above the rated power handling of the loudspeaker, and should control the duration of power levels in excess of the rating. Additionally, effective protection should seek to allow all power levels below the loudspeakers rating to pass unaltered, i.e. minimal attenuation, filtration, and distortion.
Referring to FIG. 2A, required attenuation and time duration plots of a typical loudspeaker are overlaid with a typical self-actuating, self-attenuating lamp. The lamp's attenuation is represented by the solid line, 52, and the time response of the lamp is represented by the dashed line 54. As evident in FIG. 2A, the lamp self-actuates and attenuates before the load's power handling rating, 44, and begins a linear increase in attenuation. Unfortunately, the lamps attenuation plateaus and is significantly less than what the load requires to maintain damage-free operation. Shaded region 56 illustrates the damage region in which the load would receive more power than the specified rating. The lamps time response is somewhat fast, as evident by dotted line 54, clearly faster than the initially required response time of the loudspeaker, 48. This initial excessive speed will clamp many safe transient power levels quicker than required. However, at extreme high power levels the lamp actuation and attenuation time lags behind the required response time of the loudspeaker, 48, which allows operation in the damage region, 58. Lamps also have a nominal impedance even when they are not actuated or lighting, which results in a measurable insertion loss. Finally, lamps have a maximum power rating at which the filament can be damaged upon over-powering the device, which greatly limits the operational power range of circuits that incorporate lamps without subsequent filament protection. A representative schematic of a single lamp, loudspeaker protection circuit is provided in FIG. 2B.
Referring to FIG. 2C, required attenuation and time duration plots of a typical loudspeaker transducer are overlaid with a typical self-actuating thermistor (usually a Positive Temperature Coefficient device, PTC). The PTC attenuation is represented by the solid line, 52, and the time response to reach the nominal attenuation is represented by the dashed line 54. As evident in FIG. 1B, the PTC actuates slightly before the load's power handling rating, 44, and steps quickly in attenuation. While the PTC does offer adequate attenuation, the fast-acting step attenuation response is not optimal, and when used for loudspeaker protection is easily detected by the human ear. The PTC time response is very slow, as evident by dotted line 54, and is clearly slower than the required response time of the load, 48. Shaded region 56 illustrates the damage region in which the load would receive longer power durations than the specified rating. Unfortunately, while selecting smaller PTC devices will speed the time response, the actuation threshold is typically much less than the desired power rating of the load. Additionally, PTC devices will remain actuated with a small amount of trickle current, leading to poor release and recovery performance. PTC actuation thresholds will also vary greatly depending upon the ambient temperature, greatly limiting the effective operational temperature range of circuits incorporating such devices. Because of these problems, designers have great difficulty finding a single PTC device that meets all of the desired requirements with respect to time, attenuation, actuation thresholds, and release performance. A representative schematic of a self-actuating PTC loudspeaker protection circuit is provided in FIG. 2D wherein PTC, 12, is bypassed with an optional fixed attenuator, 10.
Referring to FIG. 2E, required attenuation and time duration plots of a typical loudspeaker transducer are overlaid with a non-time delayed relay actuator and a single lamp attenuator. The attenuation characteristic, solid line 52, is the same as a single lamp; however, the lamp is not allowed to attenuate below the power rating of the load, 44. Unfortunately, the inadequate attenuation of the lamp at higher power levels remains a problem and allows operation in the damage region, 56. Due to the absence of a timing stage within the triggering circuit, the relay time response is somewhat fast, as evident by dotted line 54, clearly faster than the initially required response time of the loudspeaker, 48. This initial excessive speed will clamp many safe transient power levels quicker than required. However, at extreme high power levels the relay actuation time lags behind the required response time of the loudspeaker, 48, which allows operation in the damage region, 58. Typical electromechanical relays have a response time of 5 to 10 milliseconds, and sensitive loads may require protection less than a millisecond under these extreme high-power levels. The overall result for a non-time delayed relay circuit is a less than optimal protection topology for dynamic sensitive loads such as loudspeakers. Additionally, typical relay designs have suffered from actuation chatter wherein the relay actuates and releases rapidly when the input signal is crossing the relay coil threshold. Such chatter degrades the life of the relay contacts significantly. A representative schematic of a non-time delayed relay actuator and a single lamp attenuation circuit is provided in FIG. 2F, wherein the relay is controlled by a voltage-divided, non timed, rectified signal.
Referring to FIG. 2G, required attenuation and time duration plots of a typical loudspeaker transducer are overlaid with a time controlled, dual relay actuator, and a single lamp attenuator. The time response characteristic, line 54, is much improved over the non-timed approach presented in FIG. 2E. However, there remains a small region of damage susceptibility, 58, wherein the actuation lag is not fast enough to protect the loudspeaker from large transients. The attenuation characteristic, solid line 52, is the same as a single lamp; however, the lamp is not allowed to attenuate below the power rating of the load due to the first actuation stage threshold, line 44, and the lamp circuit is interrupted above the second actuation stage threshold, 60. The second actuator halts current flow to ensure adequate filament and load protection, but introduces excessive attenuation highlighted in the shaded region 62, wherein the load is effectively disconnected. A representative schematic of a time controlled, dual relay actuated, single lamp attenuation circuit is provided in FIG. 2H, wherein the relay is controlled by a detected, timed, rectified signal.
The final category of designs to consider are those that have incorporated circuit components, such as thyristors, metal oxide varistors, triacs and/or TVS diodes, configured to clamp over-voltage scenarios. Clamping devices are typically connected in parallel with the load allowing a current shunt to ground or neutral in the case of an overage condition. These devices, while very fast, have presented several problems to high-performance protection circuits: (1) excessive currents exist when clamping and can result in damage to the clamping device, the AC source, or passive line conditioning circuitry connected thereto; (2) clamping techniques result in non-linear loading on the AC driving device and are not acceptable for protection circuits that are required to connect to a variety of different AC sources; and (3) significant signal distortion is added when voltage clamping, or “clipping”, is active. Due to these significant problems with clamping and crow-bar designs, no effort was made to present graphical plots of their performance.
Overall, it should be well understood that none of the existing state of the art techniques for protecting sensitive AC loads have incorporated high-efficiency, digitally programmable attenuation or microprocessor based control. All previous attempts have relied upon discrete control circuitry and lossy attenuators that have fixed inherent properties, i.e. lamp filaments, resistors, thermistors. Therefore, it should be stressed that the lack of digitally programmable attenuation and microprocessor based control were two fundamental deficiencies of prior art in AC power protection of sensitive loads.
In summary, existing AC power protection circuits have suffered from the following problems: non-programmable attenuation, lossy attenuators generating excessive heat and/or light output, non-programmable thresholds and timing coefficients, high insertion loss, abrupt stepped actuation, non-linear loading, inadequate peak voltage and current protection, limited operational power range, and actuation chatter. Additionally, existing circuits designed for continuous AC attenuation have suffered from excessive power dissipation (heat), cost, limited control, magnetic core saturation problems, frequency response anomalies, and no over-power protection monitoring.